8 Execution (p. I)

8.1 General

8.1.1 The execution of jet grouting works requires knowledge and experience in this type of construction.

NOTE The high pressure employed in the jet grouting process is to generate a high speed jet to disaggregate the soil and is not intended to be applied to the surrounding soil.

8.1.2 Jet grouted column execution method

The phases of execution usually consist of:

  • — drilling a borehole of a predetermined length;
  • — introducing to the end of the borehole a monitor connected to the jet grouting string. This is unnecessary in some cases as the string and monitor are used for drilling;
  • — jetting of the disaggregating and cementing fluid(s) through the monitor, simultaneously withdrawing and rotating the rods, with pre-established withdrawal and rotational speed, pump pressure and flow rate for each fluid.

8.1.3 Jet grouted panel execution method

The phases of execution are the same as defined for jet grouted columns, with the exception that during jetting the rods are withdrawn and not rotated. Alternatively the rods can be rotated about limited angles. The resulting panel is placed in a plane on the drilling axis, or is formed by two or more sections on planes intersecting the drilling axis (Fig.1 b).

8.1.4 Alternative execution methods

If required by soil conditions, alternative execution methods may be adopted, both for column or panel processes. Among alternatives the most usual is prejetting. An element can also be executed in sequential steps: firstly the treatment for a given length from the borehole collar is completed and allowed to gain strength. Then, after redrilling the treated soil, the process is repeated at a deeper stage, and so on, until the design length of the treatment is reached.

8.1.5 Method statement

8.1.5.1 Before starting the jet grouting works, a method statement should be submitted. This method statement should contain the following information as a minimum:

  • — identification, objective and scope of the jet grouting works;
  • — soil description;
  • — shape of the jet grouted element required;
  • — jet grouting system;
  • — design of the jet grouting treatment;
  • — working procedure (drilling, jetting, sequence of execution);
  • — jet grouting parameters;
  • — materials (for drilling and jetting);
  • — precautions to be taken to avoid unacceptable settlement or heave, especially in silty and clayey soils;
  • — site installation and working areas;
  • — plant and equipment;
  • — spoil management;
  • — quality control procedures as required by the contract;
  • — the measures taken to ensure the boring accuracy;
  • — procedures regarding possible interruptions during the jetting operations;
  • — the measures to be taken to ensure that the finished level of grouting is maintained during the initial set of the jet grouted material;
  • — possible modifications of the jet grouting parameters during the works;
  • — the testing methods;
  • — working documents (drawings, reports).

8.1.5.2 For jet grouting works in geotechnical category 3 (ENV 1997-1:1994 - clause 2.1.) a method statement shall be submitted which shall include the information listed in 8.1.5.1.

8.1.6 The ranges of the jet grouting parameters usually adopted for the different systems are listed in Annex B.

8.2 Equipment

8.2.1 The jet grouting equipment usually comprises:

  • — the drilling rig;
  • — the jet grouting rig (often is also the drilling rig) provided with the jet grouting string, the monitor and the devices able to drive the jet grouting string at predetermined rotation and translation speeds;
  • — the mixing and pumping plant supplying the jet grouting fluid (or fluids);
  • — the high pressure lines connecting the jet grouting pump to the rig;
  • — equipment to monitor pressures, fluids flow rates and volumes, rate of rotation and withdrawal, depth.

8.2.2 The jet grouting equipment shall be able to perform the jet grouting operation as for the chosen process, by assuring:

  • — the translational and rotational movement of the jet grouting string with the designed speed;
  • — the supply to the jet grouting string of the fluids coming from the plant, at the required pressure and rate of flow.

8.2.3 The length of the jet grouting string, and the height of the relevant mast, should not be shorter than the length of the designed jet grouted element. If imposed by large depths or by access limitations, the string should be divided into the minimum number of elements, in order to minimise the need to interrupt the jet grouting operation.

8.2.4 The jet grouting string:

  • — for the single system, one conduit conveying the high pressure the cement mix to the monitor;
  • — for the double system, two conduits separately conveying the two fluids (air and cement mix, or water and cement mix respectively) to the monitor;
  • — for the triple system, three conduits to allow for the high pressure water, the compressed air and the cement mix to the monitor.

Multiple conduits are assembled in the same rod.

8.2.5 The monitor comprises:

  • — for the single system, one or more circular nozzles for jetting the grout. Multiple nozzles are located at the same level or at different levels, with constant mutual staggered angles;
  • — for the double (air) system, one or more double nozzles (located at the same level or at different levels, with constant mutual staggered angles) to allow the simultaneous jetting of air and grout. The air nozzle is an annulus around the circular nozzle for grout;
  • — for the double (water) system, one or more nozzles for high pressure jetting of water and one or more deeper nozzles for sending the cement mix;
  • — for the triple system, one or more double nozzles to allow the simultaneous jetting of air and water and one or more simple nozzles located at a deeper level to allow the grout injection.

In general the air-water and grout nozzles for each couple are set at 180° apart. Multiple couples are installed at constant mutual angles.

8.2.6 The jet grouting mixing and grouting plant, for the different systems, mainly comprises:

  • — for the single system: cement and other materials storage, colloidal mixing plant, agitator tanks, high pressure grout pump;
  • — for the double (air) system: as for the single fluid system plus an air compressor;
  • — for the double (water) system: as for the single fluid system plus a high pressure water pump and a grout pump;
  • — for the triple system: as for the double fluid (water) system plus an air compressor.

EN 12716:2001 Execution of special geotechnical works — Jet grouting